So, during spring break, I was extremely bored and implimented merge sort into a language that doesn’t really need it: Bash. I got inspiration to do this from seeing merge sort implimented in Prolog. As of right now, it merely sorts integers, but can sort anything, given that a compareTo method of what you want sorted is written into the merge method (bash isn’t object oriented, so it’s not really so adaptable to adaptability.) I doubt I’m the first person to do this, but it’s a nice thought experiment to see how limited languages can still allow the performance of advanced operations. I don’t guarantee that this algorithm performs in n*log(n) time as I’m not sure of the individual costs of bash operations or the cost of reading and executing this, but I tested it with 1330 numbers and it sorted them in about 35 seconds on my 1.6 Ghz laptop.

mrg $left $right;
# The two halves have been merged together with a helper method
else
echo "Usage: mrgsrt <series of numbers seperated by spaces>";
fi;
}

mrg() {
# This method sorts two sorted lists of numbers by adding the lowest of
# firstmost unsorted number into a new list until all numbers have been
# added and then returns that list.
#
# @Author: Adam Vite

0,1 and 6 are always the same, but different distros do different things with the other run levels (for example, Debian-based systems use run level 2 for GUI and full multi-user mode)

You can see what services start at what run level by looking in the directory /etc/rc#.d where # is the run level number.

The run level number can be found in /etc/inittab (id:2:initdefault: for example)
If you were to look in /etc/rc2.d, you would see files that look like:

S99program

These are usually symbolic links to files in /etc/init.d (so a change to /etc/init.d/program would change the programs script for every run level).

Start-up scripts in Linux follow the form “start-stop” (at minimum). Many have “start-stop-restart-reload-check” or possible more.

A basic start-stop script would look like this:

#!/bin/bash
case $1 in
start)
program
;;
stop)
program
;;
esac

Don’t forget

chmod +x script

This is useful to know, especially if you are dealing with a machine that may need specific commands to be run when booting or shutting down in order to function properly (for example, in the radio station we need the client machines to run dhclient at S99 in order to obtain an IP address).

These scripts are not all run at once (by default). S##program where ## is the priority of the script.

I manage a Linux router that provides internet access to tenants in an apartment building. From time to time, a user will do something that will ruin the connection.

It can be Limewire, BitTorrent or just Skype… but the end result is a latency of more than 400ms!

Recently I came across a simple way to do this on Debian/Ubuntu.

apt-get install wondershaper

The usage is simple

wondershaper eth0 4600 490

This would limit you to 4600 kbits download and 490 kbits upload.

Before you limit your speed, make sure your connection is not being heavily used and ping google.com (and a few other servers). Get a good idea of your latency (Mine is 15ms at night, and 25ms during the day).

You should find out your actual speed; I suggest speedtest.net for testing. You should also do this across several days at different times!

Let’s say you find out that you have 5800 down and 600 up. If you ping google while you’re testing your connection you are going to notice that your latency goes up.

Since you probably want to avoid this, try different values for wondershaper until your latency is a low value even when you are using all avaible bandwidth. I find that 80% of your max speed is a good value to shoot for.

I find that I can use bittorrent on multiple computers and use speedtest.net and still have a latency of around 30ms limiting a 6000/600 connection to 5000/500.

If you come across any errors when running this script (Debian has this problem for me), just comment out the line causing the error. Everything will still work.

If you would like to add a custom startup script to your Linux machine (for running ircd or an iptables script for example) the process is simple.

Create a file in /etc/init.d

touch /etc/init.d/program

chmod +x /etc/init.d/program

Edit it, and create something like this

#!/bin/bash

case $1 in

start)

sudo -u irc /home/ircd/unreal start

;;

stop)

sudo -u irc /home/ircd/unreal stop

;;

esac

These scripts are run as root, so you should probably use sudo to run programs as unprivileged users.

Now we just have to add it to our default runlevel (runlevel can be found in /etc/inittab on Debian, in Ubuntu this file does not exist as far as I know). The runlevel should be the same across all Debian (including ubuntu) systems.

ln -s /etc/init.d/program /etc/rc2.d/S20program

The S20 requires a little bit of explanation. S means Start, and 20 means to run after the 19’s but before the 21+’s. It’s simply a priority system. You can replace S with K to ‘stop’ instead of ‘start’ when the computer shuts down.

Instead of using a complex set of firewall rules, tcpwrappers provides an easy method of restricting access to certain daemons using ip addresses, host names, and ranges. While I wouldn’t reccommend on relying soley on them, they are a great tool for workstations and other machines behind a firewall.

hosts.deny is checked first; then hosts.allow. By default, if a program is not listed in either it is granted access to everyone.